WEST BOYLSTON — Jim Hall shook his head, the excitement evident on his face.

"You keep posing, baby, that's your house," he said.

It was just before 6 p.m. on a Friday and only a moment before, a bluebird had landed on the little wooden box that Mr. Hall and his father, Skip, built and installed on the grounds of the Worcester County House of Correction — a miniature bird sanctuary.

Until that moment, Mr. Hall believed the nest inside belonged to a tree swallow, a common native species known to take up the bluebirds' prime real estate. But when the bird returned to the box three more times to look at the visitors, he realized his error.

Mr. Hall, of Charlton, and his father, 81, have shared a love of bluebirds for years.

It started at the family home in Leominster, where they erected their first birdhouse and bluebirds would return year after year.

After his parents moved to the Briarwood Continuing Care Retirement Community, the father and son spent hours making about 20 birdhouses, which they sold at a church fundraiser.

"He was overjoyed," Mr. Hall said of his father.

They weren't entirely sure what to do with the few houses they had left. Briarwood was not a likely place to attract the birds, which are particular about their habitat and take a long time to settle in.

So last year Mr. Hall and his father went to the sheriff's office, where they were told not only could they keep the houses on the property, but they could get some help installing them.

In only the project's second year, Mr. Hall, who grew up in the town, was thrilled to see that a bluebird had already taken up residence.

The roughly 6-foot tall birdhouses are made with thick pine walls and a white synthetic top. A nail pulled from the top unlocks the side of the house, which can be opened to reveal the nest inside. The real key, however, is a reinforced 1¼-inch hole which — if it were ever-so-slightly larger — would allow for predators and make the house uninhabitable for the smaller bluebird species.

A vine with budding flowers has twisted its way up the galvanized pole that holds up the house in tick-infested brush with tall grass and wildflowers. It sits between a tree line and a large field where jail personnel have planted long rows of corn and birds hunt for bugs.

"This meadow is such a dream spot for so many birds," Mr. Hall said.

There are now four birdhouses on the 200-acre plot, but Mr. Hall said that years down the road there could be dozens. He said the "No Trespassing" signs make it clear that visitors are not welcome without permission.

"It has everything needed to develop a sanctuary," he said.

A birder for the last 20 years, he said he can spot up to 50 birds in just a few hours at the site. On the grounds of the House of Correction, it is almost certain the land will remain undeveloped and undisturbed for years to come.

Bluebirds have historically nested in the holes of trees that have decayed and broken, but those can be difficult to come by.

"People tend to remove a lot of those hazard trees," said Robyn Bailey, a project leader for NestWatch, a citizen science project affiliated with the Cornell Lab of Ornithology. "Generally they fall down a lot anyway."

As vibrant blue wings make them easy to spot, bluebirds are picky about where they nest, but Mr. Hall said their consistent return to one they like, year after year — something at which he and his father have marveled — makes them stand out from other species.

"People have a special relationship with bluebirds," Ms. Bailey said. Her organization has seen a boost in the number of people reporting bluebird nests in recent years, including an incredibly rare case of twin bluebirds, which lived 11 days after hatching from the same egg last year.

"They're just so pretty and just so timid," said Sheryl Farnam, a natural history guide at the Broad Meadow Brook Conservation Center and Wildlife Sanctuary in Worcester who also teaches birding programs. She said this year the sanctuary had two successful families of bluebirds fledge from the dozen birdhouses they have installed.

Later this summer, Mr. Hall and his father plan to add at least two more houses and will continue to add to them from there.

Contact Alli Knothe at allison.knothe@telegram.com. Follow her on Twitter @KnotheA